3.14 parser silently accepts invalid except-clause syntax 'except A, B:' (Python-2 form, SyntaxError on 3.0-3.13)

[jbdevprimary]

Repro

try:
    pass
except OSError, ValueError:
    pass

$ python3.14 -c "import ast; ast.parse(open('repro.py').read())"
# no error — parses successfully

Confirmed via disassembly that it compiles to the SAME bytecode as the correct except (OSError, ValueError): form (BUILD_TUPLE + CHECK_EXC_MATCH) — not the Python-2 except Type, name: binding semantics, which would be a different and even more dangerous silent misinterpretation. So functionally the 3.14 parser is currently treating an omitted-parens exception-type list as equivalent to a parenthesized tuple.

$ python3.12 -c "import ast; ast.parse(open('repro.py').read())"
  File "<unknown>", line 3
    except OSError, ValueError:
           ^^^^^^^^^^^^^^^^^^^
SyntaxError: multiple exception types must be parenthesized

3.9, 3.12, and 3.13 (tested on this machine) all correctly raise SyntaxError: multiple exception types must be parenthesized (added by bpo-32892 / the PEG parser's dedicated error message for this exact Python-2 leftover). Only 3.14 accepts it.

Reproduced across independent 3.14 builds

• Homebrew python@3.14 3.14.6 (macOS arm64)
• uv's managed cpython-3.14.0-macos-aarch64-none (python-build-standalone)
• asdf-installed 3.14.4
• Official python:3.14-slim Docker Hub image, 3.14.7 (python:3.14-slim, pulled fresh, Linux/GCC build) — rules out anything specific to this machine or macOS

All four reproduce identically.

Why this matters

This isn't purely cosmetic: ruff format (astral-sh/ruff#28928, filed as a companion issue) appears to exploit this leniency when targeting py314, actively rewriting valid except (A, B): into the unparenthesized form — producing code that is a SyntaxError on every Python version except 3.14. A tool or codebase relying on 3.14's leniency here will silently produce code broken everywhere else.

Expected

except A, B: (without parentheses, 2+ types) should continue to raise SyntaxError: multiple exception types must be parenthesized on 3.14 as it does on every prior Python 3 version, unless this is an intentional, documented grammar change for 3.14 (in which case apologies for the noise, and a pointer to the relevant PEP/what's-new entry would be very welcome — I could not find one).

[jbdevprimary]

Retracting — this is not a bug. It's PEP 758 (Final, Python 3.14, "Allow except and except* expressions without parentheses"), which I failed to check before filing. except A, B: without an as clause is intentional 3.14+ syntax meaning "catch A or B", not a leniently-accepted Python-2 remnant. Confirmed the PEP text and behavior directly (except KeyError, ValueError: correctly falls through past a KeyError handler to catch a ValueError). Apologies for the noise — should have read what's-new for 3.14 before assuming a regression.